CN110914617B

CN110914617B - Method and system for multi-sensor gas detection

Info

Publication number: CN110914617B
Application number: CN201880046332.XA
Authority: CN
Inventors: 陈磊; M.皮奇
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2017-05-12
Filing date: 2018-05-11
Publication date: 2023-01-13
Anticipated expiration: 2038-05-11
Also published as: CN110914617A; EP3622229B1; EP3622229A1; US20200166470A1; WO2018209296A1; US11604156B2

Abstract

Methods and systems are disclosed in which the electrical impedance of a metal oxide composition is measured in a plurality of sensors to detect combustible or reducing compounds, wherein at least one of the plurality of sensors operates at a temperature or includes a metal oxide composition; the temperature is different from a corresponding temperature of another of the plurality of sensors or the metal oxide composition is different from a metal oxide composition of another of the plurality of sensors.

Description

Method and system for multi-sensor gas detection

Background

Gas sensors have been used in a variety of applications such as process monitoring and control and safety monitoring. Since compounds can also be flammable or explosive, gas detection sensors have also been used for leak detection where such compounds are used or manufactured. Various types of sensors have been used or proposed, including but not limited to Metal Oxide Semiconductor (MOS) sensors, non-dispersive infrared detector (NDIR) sensors, catalytic combustion (ballistics) sensors, high temperature solid electrolytes permeable to oxygen ions, and electrochemical cells.

Sensors of the above type have been used and have met with varying degrees of success in industrial or laboratory settings where they have been employed. However, many such sensors have limitations that can affect their effectiveness in demanding new and existing applications. For example, catalytic combustion sensors are prone to false alarms due to cross-sensitivity (cross-sensitivity). NDIR sensors have been used in small volume applications, but can be difficult and expensive to manufacture to commercial tolerances. Electrochemical sensors rely on redox reactions involving test gas components at electrodes separated by an electrolyte that generates or affects an electrical current in an electrical circuit connecting the electrodes. However, solid state electrochemical sensors may be difficult to implement for some materials. For example, solid state electrochemical sensor testing for flammable hydrocarbons may utilize solid electrolytes formed from ceramics such as perovskites, which may require high temperatures (typically in excess of 500 ℃), making them impractical for many applications. Some electrochemical sensors that operate at lower temperatures (e.g., carbon monoxide sensors, hydrogen sulfide sensors) require the presence of water at the electrode/electrolyte interface for the electrochemical redox reaction, which can make them impractical for many applications.

MOS sensors rely on the interaction between a gaseous test component, such as hydrogen sulfide or a hydrocarbon, and adsorbed oxygen on the surface of a metal oxide semiconductor. In the absence of the gaseous test component, the metal oxide semiconductor adsorbs atmospheric oxygen at the surface, and this adsorbed oxygen captures free electrons from the metal oxide semiconductor material, resulting in a measurable level of base resistance of the semiconductor at a relatively high level. Upon exposure to a reducing or flammable gaseous test component, such as hydrocarbons or Hydrofluorocarbons (HFCs), the gaseous test component interacts with adsorbed oxygen such that it releases free electrons back into the semiconductor material, resulting in a measurable decrease in impedance, which may be correlated to the measured test gas component level.

In the HVAC/R industry, more environmentally friendly refrigerants are being developed and used to replace refrigerants with high Global Warming Potentials (GWPs), such as R134A and R410A. Many low GWP refrigerants are flammable (A3 refrigerants, such as R290, i.e. propane) or slightly flammable (A2L refrigerants, such as R32, R1234ze, etc.). Leak detection sensors have been proposed to address potential fire hazards from flammable refrigerants in interior building spaces. Conventional MOS sensors have been considered as low cost options for such applications. However, it has been shown that MOS sensors can be deactivated by exposure to certain volatile chemicals that may be present in commercial and residential environments. Both temporary and permanent poisoning associated with those volatile compounds may occur. US9182366 discloses a method of rapidly cycling the temperature of the sensing element of a micro-electromechanical system (MEMS) MOS sensor between high and low temperatures to vaporize contaminants. In this approach, the embedded heater will always experience thermal cycling, which can compromise the useful life of the sensor. It is not clear at all whether conventional non-MEMS can even sustain a rapid thermodynamic cycle. In addition, if degradation or accidental poisoning has substantially shortened the useful life of the sensor, it is not known whether the sensor is still operational. For devices used for security monitoring, the lack of notification of unpredictable failures can be a significant drawback. US20020168772A1 discloses a method of diagnosing whether a MOS sensor has been poisoned by modulating the temperature of the same sensing element and comparing the resistance changes thereof. This approach can temporarily interrupt the monitoring function when the diagnostic procedure is performed, and is therefore prone to risk being unavailable to detect hazardous events during sensor deployment.

In addition, even without harmful chemicals, in benign environments, the most advanced MOS sensors are able to reach the end of life in 3 to 7 years, which may still be insufficient for system requirements, such as for HVAC systems. The use of a single sensing element for both primary monitoring and poisoning diagnostics can be further problematic due to non-transient thermal responses when heater power is changed between diagnostic and normal operating modes.

Disclosure of Invention

According to some embodiments of this disclosure, a method for monitoring a combustible or reducing compound comprises: measuring an electrical impedance of the metal oxide semiconductor composition in a plurality of sensors, wherein a first sensor operates at a first operating temperature for primary monitoring of the combustible compound. The second sensor operates at a second temperature that is lower than the first temperature and higher than a temperature at which condensation of water vapor can occur.

According to some embodiments, a monitoring system for combustible or reducing compounds includes a plurality of sensors disposed in communication with a gas being monitored. The sensors each include a metal oxide semiconductor composition, an impedance measuring device, and a heater. The system also includes a controller configured to operate the plurality of sensors. The controller and the heater of the plurality of sensors are configured to operate the first sensor at a first operating temperature for primary monitoring of the combustible compound. The controller and the heater of the plurality of sensors are configured to operate the second sensor at a second temperature that is lower than the first temperature and higher than a temperature at which condensation of water vapor can occur.

In accordance with any of the above embodiments, the gas being monitored is capable of flowing through the conduit, and the first sensor and the second sensor are disposed in the conduit, wherein the second sensor is downstream of the first sensor with respect to a direction of gas flow through the conduit.

According to some embodiments, the air conditioning system comprises a first heat exchanger. A first heat exchanger includes a conditioned air flow path on a first side of the heat exchanger and a refrigerant flow path including a flammable refrigerant on a second side of the heat exchanger in thermal communication with the first thermal side. A closed fluid flow path comprising a flammable refrigerant connects the refrigerant flow path of the first heat exchanger with a second heat exchanger in thermal communication with an external heat source or heat sink. A plurality of sensors are disposed in an air space surrounding the first heat exchanger or the closed fluid flow path. The sensors each include a metal oxide semiconductor composition, an impedance measuring device, and a heater. The system also includes a controller configured to operate the plurality of sensors. The controller and the heater of the plurality of sensors are configured to operate the first sensor at a first operating temperature for primary monitoring of the combustible compound and to operate the second sensor at a second temperature that is lower than the first temperature and higher than a temperature at which condensation of water vapor can occur.

According to some embodiments, the above air conditioning system refrigerant can have a class 2 or class 2L flammability rating according to annex ak of ASHRAE 34-2007 in 2010.

In accordance with any one or combination of the above embodiments of the air conditioner system, the sensor can be disposed in a duct on the conditioned air flow path including an inlet and an outlet, and the first sensor and the second sensor are disposed in the duct, wherein the second sensor is downstream of the first sensor with respect to a direction of flow from the inlet to the outlet.

According to some embodiments, a method of operating an air conditioning system according to any one or combination of the above embodiments comprises: the air conditioning system is placed in an operational state that includes flowing refrigerant over a refrigerant flow path in response to a system demand, and periodically or continuously operating the first and second sensors to test for the presence of refrigerant on the conditioned air flow path.

The measured impedance of the first sensor can be compared to the measured impedance of the second sensor according to any one or combination of the above embodiments.

According to any one or combination of the above embodiments, the status of operation of the first sensor can be determined based on the compared impedance measurements of the first sensor and the second sensor.

According to any one or combination of the above embodiments, it is possible to change the temperature of the second sensor to a third temperature that is higher than the second temperature and is less than or equal to the first temperature, and to compare the measured impedance of the first sensor with the measured impedance of the second sensor at the third temperature.

The third temperature can be from 40 ℃ to 60 ℃ according to any one or combination of the above embodiments.

According to any one or combination of the above embodiments, the measured impedance comparison of the first sensor and the second sensor can be performed after flowing the gas without the combustible compound to the sensor.

According to any one or combination of the above embodiments, when the first sensor reaches the end of life, the operating temperature of the second sensor can be increased to the first temperature and used for primary monitoring of the combustible compound.

The first temperature can be from 300 ℃ to 500 ℃ according to any one or combination of the above embodiments.

The first second temperature can be from 85 ℃ to 130 ℃ according to any one or combination of the above embodiments.

In accordance with any one or combination of the above embodiments, the third sensor is operable at a fourth temperature that is lower than the second temperature and higher than a temperature at which condensation of water vapor can occur.

According to any one or combination of the above embodiments, the third sensor can be disposed in the conduit downstream of the second sensor with respect to the direction of flow from the inlet to the outlet, or the third sensor can be disposed in the conduit separately from the first sensor and the second sensor.

The measured impedance of the first sensor can be compared to the measured impedance of the third sensor according to any one or combination of the above embodiments.

According to any one or combination of the above embodiments, the measured impedance of the first sensor can be compared to the measured impedance of the second sensor if the first sensor has detected a combustible compound, and the measured impedance of the first sensor can be compared to the measured impedance of the third sensor if the first sensor has not detected a combustible compound.

The fourth temperature can be less than or equal to 50 ℃ and greater than the dew point of the gas being monitored, according to any one or combination of the above embodiments.

Drawings

The subject matter regarded as the disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic depiction of an exemplary embodiment of a sensor;

FIG. 2 is a schematic depiction of a system for gas monitoring with two sensors;

FIG. 3 is a sensor performance graph from an example embodiment of a two sensor system and method as disclosed herein;

FIG. 4 is a flow diagram of an example embodiment of a monitoring protocol (protocol) for the two sensor systems and methods as disclosed herein;

FIGS. 5A and 5B are schematic depictions of a first sensor, a second sensor, and a third sensor disposed in a gas flow conduit; and

FIG. 6 is a flow diagram of an example embodiment of a monitoring protocol for a three sensor system and method as disclosed herein.

Detailed Description

As mentioned above, the systems and methods described herein include a plurality of sensors that include a metal oxide composition. Various types of metal oxide sensor configurations can be used in accordance with this disclosure. In the exemplary embodiment shown in FIG. 1, the gas sensor 30 includes a gas sensing element 10 having a metal oxide semiconductor body 12 with a gas sensing surface 14 integrated with parallel or interdigitated (higher gain as shown)

electrodes

32 and 34. The sensor is configured with a doped metal oxide semiconductor at the gas sensing surface 14 disposed between the

interdigitated electrodes

32 and 34. The

electrodes

32, 34 are shown on top of the sensing element 10, but can also be provided at other locations, such as at the bottom. The electrodes are connected from outside the gas sensing element 10 to a circuit 36 that includes a signal processor 38. The signal processor 38 can be a voltmeter or an ammeter, but in many cases includes a potentiostatic circuit, a voltage divider circuit, a bridge circuit, a microprocessor, an Electronic Control Unit (ECU), or similar electronic device with integrated voltage and or amperometric measurement functionality, and can also apply a bias voltage between the

electrodes

32 and 34. A heater (not shown) can be in thermally conductive contact with the surface of the metal oxide semiconductor body 12 (e.g., attached to the bottom surface of the metal oxide semiconductor body 12) and controlled and powered by the signal processor 38. Other sensor components, including but not limited to housings, mounting hardware, gas flow conduits, fluid cavities, are not shown in fig. 3, but can be incorporated into the sensor by the technician.

In some embodiments, the metal-oxide-semiconductor sensor can be configured as an array of sensor elements on an integrated circuit chip. With respect to the multiple sensors utilized in the methods and systems disclosed herein, the multiple different components can be on different chips or on the same chip. For example, sensors operating at different temperatures can be disposed on different chips within the same sensor housing, or on different chips within different sensor housings, in order to facilitate the maintenance of different temperatures. Sensors having different semiconductor compositions can be fabricated on the same chip or on different chips and can be located in the same sensor housing or in different sensor housings. Different components of multiple sensors can be mounted at a common location on one circuit board, or can be mounted on different circuit boards, which can be positioned at multiple locations of interest for gas monitoring.

Examples of metal oxide semiconductors include, but are not limited to, aluminum (III) oxide, bismuth (III) oxide, cadmium (III) oxide, cerium (IV) oxide, chromium (III) oxide, cobalt (III) oxide, copper (II) oxide, iron (III) oxide, gallium (III) oxide, indium (III) oxide, molybdenum (VI) oxide, niobium (V) oxide, nickel (II) oxide, tantalum (V) oxide, tin (IV) oxide, titanium (IV) oxide, tungsten (VI) oxide, vanadium (5) oxide, zinc (II) oxide, and mixtures of these. Mixed metal oxides (e.g., snO) can also be utilized ₂ CuO or other mixed oxides of the above metal oxides). Transition metal dopants can be used to enhance the responsiveness of the metal-oxide-semiconductor to the target gas being sensed and to allow electrical impedance to be created between the target gas and also at the gas sensing surface 14Other gas zones of variation above. In some embodiments, the dopant is a group 5 to group 11 transition metal. Examples of transition metal dopants include copper, silver, gold, iron, ruthenium, nickel, platinum, palladium, or vanadium. While any of the above materials are capable of exhibiting a change in electrical impedance in response to exposure to a variety of test gas compositions, some materials have been used more extensively than others for particular applications. For example, copper-doped tin oxide can be used for hydrogen sulfide sensing elements, while platinum and palladium doping are typically used in sensing hydrogen or hydrocarbons. Such combinations and others are included in this disclosure. A variety of other materials can be included in the metal oxide semiconductor at the gas sensing surface 14, including but not limited to noble metals (e.g., silver, gold). Dopants, metal oxide semiconductors, other materials, and combinations thereof are disclosed in "chemiresistor gas sensors" of Kaur, m. Aswal, d.k. And Yakhmi, j.v.: materials, mechanics and manufacturing (Chemiresistor Gas Sensors: materials, mechanics and Fabrication) "Chapter 2, science and technology of Chemiresistor Gas SensorsScience and Technology of Chemiresistor Gas Sensors) Among others, ed, aswal, d.k, and Gupta, s.k, new star scientific press, new york, 2007, and in Bochenkov, v.e, and Sergeev, g.b, "Sensitivity, selectivity, and Stability of Gas-Sensitive Metal Oxide Nanostructures (Sensitivity, selectivity, and Stability of Gas-Sensitive Metal-Oxide Nanostructures, chapter 2, in Metal Oxide Nanostructures and uses thereof: (Metal Oxide Nanostructures and Their Applications) U.S. scientific press, california, 2010, each of these publications is incorporated herein by reference in its entirety.

As mentioned above, in some embodiments, the system for monitoring combustible or reducing compounds can include at least one sensor operating at a different temperature than another sensor. An example embodiment of a heat transfer system with an integrated sensor for monitoring escaping heat transfer fluid is shown in fig. 2. As shown in fig. 2, the heat transfer system includes a compressor 10 that pressurizes a refrigerant or heat transfer fluid in its gaseous state, which not only heats the fluid but also provides pressure to circulate it throughout the system. The hot pressurized gaseous heat transfer fluid exiting from compressor 10 flows through conduit 15 to heat discharge heat exchanger 20, which heat discharge heat exchanger 20 acts as a heat exchanger to transfer heat from the heat transfer fluid to the ambient environment, resulting in condensation of the hot gaseous heat transfer fluid to the pressurized intermediate-temperature liquid. The liquid heat transfer fluid exiting from the heat discharge heat exchanger 20 (e.g., condenser 20) flows through conduit 25 to expansion valve 30, where the pressure is reduced. The reduced pressure liquid heat transfer fluid exiting the expansion valve 30 flows to a fan coil unit 35 within a building 37 that includes a fan 38 and a heat absorbing heat exchanger 40 (e.g., an evaporator), the heat absorbing heat exchanger 40 acting as a heat exchanger that absorbs heat from the surrounding environment and boils the heat transfer fluid. In the heat absorbing heat exchanger 40, heat is absorbed by the refrigerant from a conditioned air flow path including a return air duct 42 returning air from a conditioned air space within the building 37 and a supply air duct 44 supplying conditioned air to the conditioned air space within the building 37. The gaseous heat transfer fluid exiting heat discharge heat exchanger 40 flows through conduit 45 to compressor 10, completing the heat transfer fluid circuit. The heat transfer system is capable of transferring heat from the environment surrounding the evaporator 40 to the environment surrounding the heat discharge heat exchanger 20. The thermodynamic properties of the heat transfer fluid allow it to reach a sufficiently high temperature when compressed such that it is greater than the environment surrounding the condenser 20, allowing heat to be transferred to the surrounding environment. The thermodynamic properties of the heat transfer fluid should also have a boiling point at its post-expansion pressure, which allows the environment surrounding the heat discharge heat exchanger 20 to provide heat at a temperature to vaporize the liquid heat transfer fluid.

As further shown in fig. 2, the heat transfer system further includes a sensor pack 50, which is shown in greater detail in the enlarged projection shown in fig. 2, wherein

sensors

52 and 54 are disposed in a conduit 56 having an inlet 58 and an outlet 59. The inlet 58 and outlet 59 are open so that the

sensors

52 and 54 will be exposed to flammable refrigerant vapor from any leaks. During operation of the fan 38, air moves through the duct 56 in the direction of arrow 60. In some embodiments, placing the second sensor 54 downstream of the first sensor 52 can provide the following technical effects: in the event of exposure to a flammable compound, the second sensor 54 receives dilution air after the fan is activated by detection by the upstream sensor 52 before the vapor front can reach the downstream sensor 54. The sensors communicate with a controller, such as an electronic control unit (ECU, now shown), which is capable of providing power to the sensors, directing control of the sensors, and receiving data from the sensors. In some embodiments, sensor 52 can perform a primary monitoring function, and sensor 54 can perform a reference function, a diagnostic function, a backup monitoring function, or any combination of these functions. Of course, the location and function of the sensors described are specific example embodiments, and other configurations can be used in which any of a plurality of sensors can perform a primary monitoring function, a reference function, a diagnostic function, a backup monitoring function, or any combination of these functions.

As mentioned above, in some embodiments, at least one of the plurality of sensors operates at a different temperature than another of the plurality of sensors. In some embodiments, this can provide the technical effect of allowing one sensor to operate at a higher temperature in the primary monitoring mode, while the other sensor operates at a lower temperature where it can have a lower sensitivity to adverse effects such as loss of sensitivity. In some embodiments, sensors operating at lower temperatures can operate in a secondary monitoring mode, a reference mode, a diagnostic mode, in a backup monitoring mode, or a combination of any of these modes, as described in additional detail below.

An example embodiment of a method and system having two sensors operating at different temperatures is described below with respect to FIG. 3. FIG. 3 shows the temperature T at a first temperature in the main monitoring mode _O Reference resistance R of the first sensor operating below (curve 1) _O Graph over time. In some embodiments, T _O Can be arranged onHaving a lower limit of 200 ℃, 250 ℃, 275 ℃ or 300 ℃ and having an upper limit of 350 ℃, 400 ℃, 450 ℃ or 500 ℃. The second sensor (curve 2) is less than T _O Second temperature T _P The following operations are carried out. In some embodiments, T _P Can be in a range having a lower limit of 70 ℃, 75 ℃, 80 ℃ or 85 ℃, and an upper limit of 100 ℃, 110 ℃, 120 ℃ or 130 ℃. As shown in fig. 3, at various points at time 4 during the service period, the system can initiate a diagnostic procedure, for example, in response to a period of time elapsed since a prior diagnostic procedure or in response to detection of a combustible compound by the first sensor 1. As described in more detail below (fig. 3), diagnostics can involve flowing clean air to the sensor. The flow of clean air (i.e., free of combustible compounds) can be initiated by, for example, flowing room air along the conditioned air flow path by operating fan 38 (fig. 2) alone, or in cases where monitoring is done outside of the system air flow path, a dedicated fan for the sensor (e.g., a fan integrated with duct 56) can be used. As shown in fig. 2, the impedance of the first sensor shown in curve 1 drops at point 4 from exposure to the combustible compound(s) (in this case, the protocol is assumed to be initiated by the drop in impedance measured by the first sensor), and then recovers as the fan purges the sensor with clean air. The operating temperature of the second sensor increases To a temperature approximately equal To, resulting in a drop in the measured impedance of the second sensor, as shown in curve 2. The measured impedance of the sensor (denoted Δ R in FIG. 3) _O ) The comparison of (b) can be made in clean air, with the second sensor at its elevated temperature, and the state of health of the first sensor can be determined based on the comparison. When Δ R is _O Beyond a specified value, the first sensor can exit as shown by the interruption of curve 1 at point 5. In some embodiments, the second sensor can then be operated at the new higher temperature To, as represented by curve 6. During monitoring, the impedance of the sensor can be measured continuously or in repeated pulses, which means that in some embodiments, metal oxygenThe compound semiconductor composition is adapted to a certain level of continuous or repeated current during monitoring. It should be noted herein that the time scale on the x-axis of the graph of fig. 3 is not limited to any particular numerical scale, and in some embodiments, monitoring can occur over extended periods up to and including infinite periods, where monitoring can occur indefinitely until a change in a status event, such as a loss of power, a service event, the end of sensor life, or the occurrence of an alarm condition. In some embodiments, monitoring can extend up to 10 years.

The protocol for performing the diagnostic procedure at point 4 in fig. 3 is shown in more detail in fig. 4. As shown in FIG. 4, initiation of the algorithm routine is represented by block 100, and from block 100 the routine proceeds to decision block 102, wherein an inquiry is made as to whether the measured impedance R1 of the first sensor is less than the threshold Ra. If R1 < Ra, system mitigation is actuated at block 104 by turning on fan 38 (FIG. 2). If R1 is not less than Ra, the routine proceeds to decision block 106, where an inquiry is made as to whether the first sensor, which is operating as a test sensor, has checked for impedance over a recently defined period (e.g., 1 month). If the most recent check has been performed, the program can optionally return to decision block 2 to recheck if R1 < Ra, or return to normal operation at block 122 if a recheck has been performed or not used. If the most recent impedance check has not been performed, the program advances to block 108 where the fan is activated to create clean air at the sensor. The fan actuation at block 108 can be different from the fan actuation for system mitigation of combustible vapors (e.g., lower fan speed or shorter duration) that was initiated at block 104. In either case, however, the routine proceeds to block 110, where the second sensor is activated by increasing its temperature to T _O For a predetermined period (e.g., 30 minutes) and measure its impedance R2. The process advances to decision block 112 where an inquiry is made as to whether the difference between R1 and R2 is greater than a specified value Δ R _C . If R1-R2>ΔR _C Then the routine proceeds to block 114 which initiates a protocol to maintain the temperature of the second sensor at T _O And the second sensor as a new transmissionSensor #1 operates. If the difference between R1 and R2 is not greater than Δ R _C Then the routine proceeds to a block 118 where the R1 measurement is updated, and to a decision block 120 where an inquiry is made as to whether the first sensor has operated beyond its expected life span (e.g., 5 years). If the first sensor has not exceeded its expected lifespan, the system returns to normal operation at block 122. If the first sensor has exceeded its expected life, the program proceeds to block 114 to monitor the transition to the second sensor. At block 116, the changed signal is sent to the main system controller, enabling the installation of a new sensor #2 and the start of a new sensor life timer for the new sensor # 1. After the new sensor is installed, the process returns from block 116 to decision block 102 for a new diagnosis.

It should be noted that although the above embodiments are described with a single pair of sensors, multiple pairs configured in the same manner (e.g., at different locations) can be used. Also, the sensor can be employed in a plurality of sensors greater than two sensors. For example, in some embodiments, a third sensor can be utilized, as shown in fig. 5-6. In some embodiments, can be less than T _P Temperature T of _R The third sensor is operated. In some embodiments, T _R Can be in a range having a lower limit of an expected environmental expiration point (e.g., 32 ℃, 36 ℃, or 40 ℃) and an upper limit of 45 ℃, 50 ℃, 55 ℃, or 60 ℃. As shown in fig. 5A and 5B, a third sensor 55 can be provided in a duct 56 for use in a system such as the air conditioning system shown in fig. 2. Fig. 5A and 5B use the same part numbering scheme as the tubing 56 shown in the enlarged sensor pack 50 of fig. 2, which need not be repeated here. As shown in fig. 5A, the third sensor 55 is separated from the first and second sensors 52/54 by a 180 ° bend in the conduit. As shown in fig. 5B, during normal operation, the third sensor 55 is separated from the first and second sensors 52/54 by a damper 62, wherein a vent 64 provides an outlet for gas flow when the outlet 59 is isolated by the damper 62 in a closed position. In some embodiments, the separation or isolation of the third sensor 55 can provide protection for the third sensorThe sensor is protected from the technical effects of exposure to chemical species that can contaminate the sensor.

The protocol for a diagnostic procedure with three sensors is shown in fig. 6. As shown in FIG. 6, initiation of the algorithm routine is represented by block 100, and from block 100 the routine advances to decision block 102, where an inquiry is made as to whether the measured impedance R1 of the first sensor is less than the threshold Ra. If R1 < Ra, system mitigation is actuated at block 104 by turning on fan 38 (FIG. 2). If R1 is not less than Ra, the routine proceeds to decision block 106, where an inquiry is made as to whether the first sensor, which is operating as a test sensor, has checked for impedance over a recently defined period (e.g., 1 month). If the most recent check has been performed, the program can optionally return to decision block 2 to recheck if R1 < Ra, or return to normal operation at block 122 if a recheck has been performed or not used. If the most recent impedance check has not been performed, the program advances to block 124 where the fan is actuated to create clean air at the sensor, if present, the damper 62 (FIG. 5B) is opened, and the temperature T is increased by heating to the operating temperature _O A period of time (e.g., 30 minutes) activates the third sensor 55 (fig. 5A/5B) and its impedance R3 is measured. The fan actuation at block 124 can be different (e.g., lower fan speed or shorter duration) than the fan actuation for system mitigation of combustible vapors that was undertaken at block 104.

Fan operation at Block 104 causes the first sensor to pass R1<In the event of a trip by Ra, the routine proceeds from block 104 to block 110, where the second sensor increases its temperature by increasing its temperature to T _O For a predetermined period (e.g., 30 minutes) and measure its impedance R2. The routine then proceeds to decision block 112, where an inquiry is made whether the difference between R1 and R2 is greater than a specified value Δ R _C . If R1-R2>ΔR _C Then the routine proceeds to block 114 which initiates a protocol to maintain the temperature of the second sensor at T _O Where the second sensor is operated as a new sensor #1, followed by block 116 where the changed signal is sent to the main system controller so that a new sensor #2 can be installed and is newSensor #1 starts a new sensor life timer. After the new sensor is installed, the process returns from block 116 to decision block 102 for a new diagnosis. If the difference between R1 and R2 is not greater than Δ R _C Then the routine proceeds to a block 118 where the R1 measurement is updated, and then to a decision block 128 where an inquiry is performed as to whether the first sensor has operated beyond its expected lifespan (e.g., 5 years). If the first sensor has not exceeded its expected lifespan, the system returns to normal operation at block 122. If the first sensor has exceeded its expected life, the program proceeds to block 116 to send a notification of the expiration of the sensor's useful life to the system controller. This portion of the routine of FIG. 6 represents that alternative embodiments can be used with two sensors, three sensors, or other numbers of sensors, as compared to FIG. 4, where sensor life expiration merely results in a notification if the primary sensor otherwise passes through the diagnostic routine.

In the event that the fan is operated at block 124 and the third sensor 56 is actuated because the first sensor impedance has not been recently checked, the routine proceeds from block 124 to decision block 126, where a query is made as to whether R1-R3 are present>ΔR _C (or if the second sensor has previously replaced the first sensor for primary monitoring, an inquiry is made as to whether R2-R3 is present>ΔR _C . If R1 (or R2) -R3>Δ RC, then the routine proceeds to block 114, which initiates a protocol to maintain the temperature of the second sensor at T _O And the second sensor is operated as new sensor # 1. If the difference between R1 (or R2) and R3 is not greater than Δ RC, the process proceeds to block 122 to resume normal operation.

While the disclosure has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the disclosure is not limited to such disclosed embodiments. Rather, the disclosure can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the disclosure. Additionally, while various embodiments of the disclosure have been described, it is to be understood that aspects of the disclosure may include only some of the described embodiments. Accordingly, the disclosure is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

Claims

1. A method for monitoring combustible or reducing compounds comprising measuring the electrical impedance of metal oxide constituents in a plurality of sensors, wherein a first sensor operates at a first operating temperature as the primary monitoring for combustible compounds and a second sensor operates at a second temperature, lower than the first temperature and higher than a temperature at which condensation of water vapour can occur.

2. A monitoring system for combustible or reducing compounds, comprising:

a plurality of sensors each comprising a metal oxide semiconductor composition disposed in communication with a gas being monitored, an impedance measuring device, and a heater; and

a controller configured to operate the plurality of sensors; wherein

The controller and the heater of the plurality of sensors are configured to operate a first sensor at a first operating temperature for primary monitoring of combustible compounds and a second sensor at a second temperature, the second temperature being lower than the first temperature and higher than a temperature at which condensation of water vapor can occur.

3. The method of claim 1, wherein the gas being monitored flows through a conduit and the first sensor and the second sensor are disposed in the conduit, wherein the second sensor is downstream of the first sensor relative to a direction of gas flow through the conduit.

4. An air conditioning system comprising:

a first heat exchanger comprising a conditioned air flow path on a first side of the heat exchanger and a refrigerant flow path comprising a flammable refrigerant on a second side of the heat exchanger in thermal communication with the first thermal side;

a closed fluid flow path comprising a flammable refrigerant connecting the refrigerant flow path of the first heat exchanger with a second heat exchanger in thermal communication with an external heat source or heat sink;

a plurality of sensors each comprising a metal oxide semiconductor composition, an impedance measurement device, and a heater disposed in an air space surrounding the first heat exchanger or the enclosed fluid flow path; and

a controller configured to operate the plurality of sensors; wherein

5. The system of claim 4, wherein the refrigerant has a class 2 or class 2L flammability rating according to appendix ak of ASHRAE 34-2007 in 2010.

6. The system of claim 4 or 5, wherein the sensor is disposed in a duct on the conditioned air flow path including an inlet and an outlet, and the first sensor and the second sensor are disposed in the duct, wherein the second sensor is downstream of the first sensor with respect to a direction of flow from the inlet to the outlet.

7. A method of operating the air conditioning system of any of claims 4-6, comprising: placing an air conditioning system in an operational state that includes flowing refrigerant over a refrigerant flow path in response to a system demand and periodically or continuously operating the first and second sensors to test for the presence of refrigerant on the conditioned air flow path.

8. The method of claim 1, wherein the measured impedance of the first sensor is compared to the measured impedance of the second sensor.

9. The method of claim 8, wherein the status of operation of the first sensor is determined based on a compared impedance measurement of the first sensor and the second sensor.

10. The method of claim 8 or 9, wherein the temperature of the second sensor is changed to a third temperature, the third temperature being higher than the second temperature and less than or equal to the first temperature, and the measured impedance of the first sensor is compared to the measured impedance of the second sensor at the third temperature.

11. The method of claim 10, wherein the third temperature is from 40 ℃ to 60 ℃.

12. The method of claim 7, wherein the measured impedance comparison of the first sensor and the second sensor is performed after flowing gas free of combustible compounds to the sensor.

13. The method of claim 1, wherein the operating temperature of the second sensor is increased to the first temperature when the first sensor reaches the end of life and is used for primary monitoring of combustible compounds.

14. The method of claim 1, wherein the first temperature is from 300 ℃ to 500 ℃.

15. The method of claim 1, wherein the second temperature is from 85 ℃ to 130 ℃.

16. The method of claim 1, wherein the third sensor is operated at a fourth temperature that is lower than the second temperature and higher than a temperature at which condensation of water vapor can occur.

17. The method of claim 16, wherein the third sensor is disposed in the pipe downstream of the second sensor with respect to the direction of flow from the inlet to the outlet, or wherein the third sensor is disposed in the pipe in isolation from the first sensor and the second sensor.

18. The method of claim 16 or 17, wherein the measured impedance of the first sensor is compared to the measured impedance of the third sensor.

19. The method of claim 18, wherein if the first sensor has detected a combustible compound, comparing the measured impedance of the first sensor to the measured impedance of the second sensor, and if the first sensor has not detected a combustible compound, comparing the measured impedance of the first sensor to the measured impedance of the third sensor.

20. The method of any of claims 16-17, wherein the third temperature is less than or equal to 50 ℃ and greater than a dew point of the gas being monitored.